Hybrid rolling bearings are often used in very demanding environments, for example in applications with reduced lubrication conditions and/or in high speed and/or high temperature applications. With the increased severity of the working conditions, e.g. heavier loads in combination with higher temperatures, thinner lubrication films and/or poor lubrication conditions the bearing components can suffer from surface initiated fatigue, so called micropitting. Even if micropitting is not necessarily a primary failure mode, it can facilitate/accelerate the appearance of other failures like indentation, surface initiated spalling and seizure.
Thus, micropitting is one of the mechanisms responsible for life-limiting bearing wear. One approach to mitigate the effects of micropitting is to ensure that the rolling contact surfaces in a bearing are always separated by a lubrication film of sufficient thickness.
This is not possible in ultra-thin lubrication film thickness (UTFT) applications. UTFT applications in rolling bearings refer to conditions when the separation of surfaces by a lubricating film is compromised by:
(i) low viscosity of the lubricant i.e. fluids with a dynamic viscosity lower than 1 cST and/or
(ii) lubricant starvation i.e. a condition where the available lubricant layer in the rolling contact inlet cannot guarantee fully flooded conditions in the bearing.
In both conditions the overall lubricant layer thickness at the contact surfaces is limited to 300 nm or less. This can happen because the bearing is lubricated with grease (limited lubricant release) or the lubricant evaporates before reaching the contact (volatile fluids) or there is limited lubricant supply by the lubrication system.
Additionally, many of these UTFT applications use media lubrication, like pure refrigerant lubrication, oil-refrigerant mixture lubrication, fuels (kerosene, diesel, gasoline, natural gas, alcohols) lubrication, and/or grease combined with media lubrication. Water lubrication is excluded. For pure refrigerant lubrications, the resulting lubricant film thickness is even significantly less than 300 nm, typically in the range of 30 nm.
The main failure mode of these rolling bearings is wear assisted by corrosion. Wear due to solid-to-solid contact enhanced by corrosion can modify the raceway profile, increase the clearance and concentrates local stresses that could develop spalls. Another important failure mode of these bearings is solid contamination. Since these applications work with very thin film thicknesses (e.g. less than 300 to 200 nm) any solid particle (debris, sand, oil soot, etc.) even the very small ones can produce damage in the contact surfaces and can modify the topography disrupting the film build-up capability of the original surface. Excessive contamination can also generate high friction forces that will hinder/block the rotation of the bearing and can produce fractures in the cage or seizure in the raceways and rolling elements.
Therefore, it has been proposed in the state of the art to employ surface engineering techniques and to provide a roughness for the raceways of the bearing rings and a roughness of the rolling elements which are as equal as possible, in order to reduce micropitting and improve the wear and fatigue life of bearings. This is based on the understanding that a rougher rolling contact surface imposes load micro cycles on a smoother, opposing rolling contact surface, in the presence of sliding and in the absence of full-film lubrication. Dis-advantageously, in practice, even in ordinary steel-steel bearings, the raceways of a bearing are generally somewhat rougher than the rolling elements. In hybrid rolling bearings the difference between the roughnesses is even greater.
It is therefore object of the present invention to provide a hybrid rolling bearing which may be used in ultra-thin lubrication film thickness applications, particularly in a refrigerant compressor device, and which has an improved corrosion and micropitting resistance.